Recovery of hydrocarbons from oil-bearing strata



United States Patent RECOVERY OF HYDROCARBONS FROM OIL-BEARING STRATAPaul G. Carpenter, BartlesviHe, Okla, assignor to Phillips PetroleumCompany, a corporation of Delaware No Drawing. Application March 28,1955 Serial No. 497,421

Claims. (Cl. 166-11) This invention relates to a process for therecovery of hydrocarbons from an oil-bearing earth formation or stratum.A specific aspect of the invention is concerned with secondary oilrecovery.

The recovery of oil from a subsurface formation may be accomplished byvarious means. The oil may be initially produced by natural means suchas gas or rock pressure, natural water drive, or solution gas pressure.When these natural forces are insuflicient to cause the oil to flow fromproducing wells at an economical rate, the oil may be pumped or raisedby any desired known method. When pumping methods are no longereffective in producing oil from a particular formation at an economicalrate, additional oil may be recovered by the employment of secondaryrecovery methods, e.g., by water flooding or repressuring the partiallydepleted sands. These methods involve the injection of water, gas, air,or a combination of these into the formation through one or more inputwells and the withdrawal of oil through one or more output or producingwells. It is also the practice in some instances to produce artificialfractures in the producing formation so as to open up the same to flowof additional oil therefrom. It has also been known to burn a fuel gasin the formation surrounding a bore hole so as to cause heavy andsemi-solid to solid hydrocarbons (kerogen) in the formation to becomesufliciently fluid to flow out. This procedure is disclosed in the Us.patent of Frederick E. Frey, 2,382,471. The technique of hydrafracing isdisclosed in copending application of I. W. Marx and H. Parker, SerialNo. 473,238. filed December 6, 1954, now Patent 2,813,583. In saidapplication, superheated steam and/or hot water are injectedinto theartificial fractures to heat up the formation and drive additional oiltherefrom.

In the secondary recovery of hydrocarbons from subsurface strata by themethods known in the art the ultimate recovery of oil has not been asgreat as desired and, of course, any oil remaining in the oil-bearingformation after secondary recovery methods have been terminated resultsin economic waste and loss which is measured by the amount of valuableoil remaining in the formation. 1 have devised a method which iseflfective in increasing the ultimate recovery of oil from an oilbearingsubsurface formation.

The principal object'of the invention is to provide a process for therecovery of oil from a subsurface oilbearing formation which requires anartificial driving force to cause the oil to flow out of the formation.Another object is to provide a process for rendering the hydrocarbons inan oil-bearing formation more fluid so that they flow out of theformation more readily. Another object is to provide a process forincreasing the ultimate recovery of hydrocarbons from ahydrocarbonbearing stratum. A further object of the invention is totaining the same. It is also an object of the invention to ice provide amethod of distilling and cracking hydrocarbons in strata containing thesame so as to effectively drive the distilled and cracked hydrocarbonstherefrom. Other objects of the invention will become apparent from aconsideration of the accompanying disclosure.

The invention comprises forming artificial fractures in an oil-bearingstratum followed by burning a combustible material in the artificialcrevices so formed so as to heat up the formation and render thehydrocarbons therein more fluid, followed by driving (or otherwiseflowing) the same out of the formation into output wells where they arerecovered by conventional means.

Hydraulic fracturing to produce artificial cracks or crevices in aformation may be effected by any suitable means, such as theconventional methods of the art, wherein a heavy fluid such as gelledliquid hydrocarbon (napalm) is injected into the formation around thebore hole with sufiicient pressure to fracture the formation andthereafter the gelled liquid hydrocarbon (usually gelled gasoline) isrendered more fluid by the heat of the formation or by the injectioninto the bore hole and the gelled gasoline in the artificial fracturesof a gel-breaking agent or peptizer which converts the gelled materialinto a readily flowable liquid. Hydraulic fracturing is also effectedwith other suitable heavy liquids such as heavy crude and/or refinedoil. In most instances, it is desirable to introduce a propping agent,such as suspended coarse sand, in the fracturing liquid to hold thecracks open after release of pressure. Any hard solid particulatematerial may be used as a propping agent. A recent technique entailsvery rapidly injecting a large volume of heavy oil in a short period oftime and this technique has met outstanding success. Sand is used in thelast batch of oil injected to hold open the cracks after release ofpressure. The same technique can be used after effecting fracturing withgelled gasoline. In the hydraulic fracturing method extremely highpressure is built up in the bore hole utilizing any desired number ofpumps working simultaneously to produce the same. Actual fracturing ofthe formation is accompanied by a sudden drop in pressure which isreadily readable on the pressure gauges connected with the injectionapparatus.

The process of the invention involves the formation of horizontalfractures in the oil-bearing stratum being produced which extend from aninput well to one or more output wells usually positioned in a generallycircular pattern around the input well. It has been found that thehydraulic fluid utilized in fracturing a stratum has been recovered inthe surrounding output wells at a distance of ,41 mile from theinjection well Within 24 hours from the fracturing operation. Thespacing of the output wells from the input well or wells is in the rangeof about 3 8 to /2 mile depending upon the character of the formationthrough which the bore holes extend. It may be advantageous to space thewells even closer or more remote in formations which warrant suchspacing.

After fracturing has been effected so that artificial cracks extendgenerally horizontally through the formation from one well to another, acombustible material is burned in the crevices so as to heat up theformation and thereby fluidize and drive the oil out of the formationinto the crevices from which it flows to the output well or wells. Theburning may be initiated and conducted as dislosed in theabove-identified Frey patent or by any other suitable means whereby theheat of combustion is imparted to the rock or sand formation above andbelow the artificial cracks in which the fluid is burned. A mixture ofnatural gas or other fuel gas and air or oxygen may be utilized as thecombustible material. It is also feasible to introduce air at therequired temperature for combustion and burn fluid hydrocarbon presentin the cracks in the formation after fracturing, thereby utilizing thenatural material in the formation for the heating process. One techniqueused is to conduct the burning step for a given period and follow thiswith fluid pressure drive through the input well to recover theresulting more fluid hydrocarbon in the surrounding output wells andfollow this procedure with another burning step to impart additionalheat to the formation and thereby heat up and/or crack additionalhydrocarbon from more remote parts of the formation.

A special advantage is obtained when using a gelled gasoline or othergelled hydrocarbon of this character in that this fracturing fluid orsemi-fluid provides an excellent combustible material for the burningstep and requires only the addition of hot oxygen or an oxygencontaininggas, such as air, to effect the burning. Any viscous combustible fluidwhich is rendered less viscous when subjected to formation heat and/orthe effects of thinning agents are adapted to use in the invention. Inoperations where it is desired to utilize alternate burning andproducing or recovery steps, gelled gasoline may be utilized in thefracturing and first burning step and also in the succeeding burningsteps but it is also feasible to utilize in succeeding burning stepsanother type of combustible material, such as natural gas or the naturalhydrocarbon present in the cracks, as the combustible material.

The fracturing and burning procedure of the invention may beadvantageously combined with a burning step prior to the fracturingwhereby oil and solid hydrocarbon in the formation surrounding the borehole is rendered more fluid so that it can be readily recovered from thebore hole in conventional manner. Following this preliminary recoverystep, the formation is fractured and the procedure of the invention isfollowed with burning of combustible material in the artificial crevicesformed by the fracturing step, followed by fluid drive of the more fluidhydrocarbons (resulting from the burning step) to the surrounding outputwells. The invention, as can be readily seen, is applicable to theproduction or recovery of hydrocarbon material from wells in which aconventional burning procedure has been utilized so as to recoveradditional hydrocarbon therefrom and increase the ultimate yield ofhydrocarbon from the formation.

Another embodiment of the invention entails forcing coarse grit into theartificial fractures or cracks at any stage in the process hereinbeforedescribed, after the initial burning step. The function of the coarsegrit forced into the cracks in the formation is to render these cracksless permeable and to render the permeability of the formation tohorizontal flow more nearly uniform so that fluid drive through theformation, rendered more permeable by the burning step, is moreeffective in driving the more fluid oil out of the entire cross sectionof the formation. The coarse grit injected into the artificial cracks inthe formation should be sufficiently large to avoid plugging of thepores of the formation and, of course, not too large to enter theartificial cracks therein. The coarseness of the grit to be used in anygiven application of the process depends upon the natural pore size ofthe formation and the thickness of the cracks. Any hard solid granularmaterial, such as sand or rock particles, may be utilized for thispurpose.

The following example is presented as illustrative of the invention andis not to be construed as unnecessarily limiting the invention.

Example A fracturing fluid is made up by adding about 3-8% by weight ofa hydroxy calcium soap to about 20-30' barrels of crude oil. The hydroxysoap comprises 40-50% of a calcium laurate or a saturated fatty acidsoap and 60-50% of a calcium naphthenate. A flow type mixer is used atthe well head and the soap and oil are mixed to produce a gel. Whensubstantially the maximum gelation of the hydrocarbon has been obtained,it is pumped into a confined zone of the well where the fracture is tobe produced, i.e., a zone which has been isolated by one or more wellpackers.

The actual type and amounts of fracturing fluid to be used will varywith the size of fracture desired, type of formation, etc. In a wellabout 4000 feet deep in a sandy formation the above fracturing fluidfunctions adequately. When the viscosity of the gel reaches 200-300centipoises, the fluid is injected into the wall at a rate of about 4barrels a minute. Sand, in the amount of 0.1 pound per gallon of liquid,is injected into the fracture fluid prior to its entry into the well.The sand serves as a prop to hold the fracture open after the pressureis released. After injection of the fracture fluid, there is pumped intothe well several barrels of oil to separate the fracturing fluid and thegel breaker which is to follow. This gel breaker consists of 20 barrelsof gasoline containing 30 gallons of 60% oil-soluble sulfonates and 40%aromatic petroleum solvents. Following the gel breaker there is pumpedinto the formation an additional 20-30 barrels of crude oil to displacethe gel breaker from the tubing into the well formation.

When the viscous fluid reaches the formation a substantial increase inthe pressure will be noted on the surface. The pressure rises until thebreakdown pressure is reached, indicating fracturing has occurred. Inthe sandy formation described this pressure is 2000-4000 p.s.i.g. Thedepth of the well, nature of the formation, folding of the formation andthe like will cause the breakthrough pressure to vary.

After a passage has been established from the input to the output well,a thermite bomb is dropped down the inlet well followed by air at 50-200p.s.i.g. and combustion is initiated. The small amount of oil andfracturing fluid in the fractured area burn and the hot combustion gasesare removed at the outlet well. The burning is continued with about 400M c.f.d. (1,000 cu. ft. per day.) of a mixture of 10% fuel gas and airat 50 p.s.i.g. The formation is thereby heated so that the oil in areasadjacent the fracture becomes more fluid and flows to the outlet well.In this manner the temperature through the fracture reaches about 11'00F. The temperature of the adjacent unburned area farthest from thefracture in a 25 foot sand thickness reaches a temperature in the rangeof -300 F. By increasing the temperature of a typical 14 API crude oilfrom 100 to F. its viscosity drops from about 630 to 67 centipoises.This great change in viscosity permits higher oil recoveries.

After one year of heating, the plug in the inlet Well is drilled out andnatural gas injected into the formation at a pressure of about 200p.s.i.g. This gives an additional drive to the oil in the heated areaand after 10 years the anticipated oil recovery would be 3,'-000-10,'000barrels per acre. In certain cases it may be advantageous to use wateras the driving fluid rather than *gas.

The specific technique to be applied to the recovery of oil from anygiven formation depends upon 'the'character of the formation as may bereadily determined by conventional procedures, such as core drilling andthe like. Some of the factors involved in'determining the particularprocedure to be used include permeability, porosity and pore size,kerogen content and specific gravity of the oil, thickness of theformation, formation temperature and pressure, concentration of connatewater, etc.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessa'ry limitations on the invention.

I claim:

1. A process for fluidizing and driving hydrocarbons from an oil bearingformation penetrated by an input well and an output well comprisingforming fractures connecting said wells; propping said fractures open;introducing and depositing a liquid fuel substantially thruout thefractures in the fractured formation; and burning said fuel thruout saidfractures by contacting same with air at ignition temperatures injectedthru said input well so as to heat the adjacent formation above andbelow said fractures, thereby rendering the hydrocarbons therein morefluid and driving same into said output well; and recovering producedhydrocarbons from said output well.

2. The process of claim 1 wherein said fuel comprises a gelled liquidhydrocarbon.

3. The process of claim 1 wherein a mixture of normally gaseoushydrocarbon and air is burned in said fractures after substantiallycomplete consumption of said fuel so as to continue the heating of saidformation and producing of hydrocarbons therefrom.

4. The process of claim 1 wherein, after the burning step, a grittyparticulate material is injected into said fractures to decrease thepermeability thereof, the particle size of said gritty material beingtoo large to clog the pores of the surrounding formation; and includingsubjecting the resulting formation to fluid drive to recover additionalhydrocarbons.

5. The process of claim 1 wherein said formation is fractured by forcinggelled hydrocarbon into same under fracturing pressure, whereby saidhydrocarbon is distributed thruout the fractures to provide said fuel.

References Cited in the file of this patent UNITED STATES PATENTS Re.23,733 Farris Nov. 10, 1953 2,272,673 Kennedy Feb. 10, 1942 2,596,844Clark May 13, 1952 2,642,943 Smith June 23, 1953 2,780,449 Fisher et alFeb. 5, 1957 OTHER REFERENCES Development of Subsurface CombustionDrive, by Hester et al., The Petroleum Engineer, November 1954, pp. B-89and 3-90.

1. A PROCESS FOR FLUIDIZING AND DRIVING HYDROCARBONS FROM AN OIL BEARINGFORMATION PENETRATED BY AN INPUT WELL AND AN OUTPUT WELL COMPRISINGFORMING FRACTURES CONNECTING SAID WELLS; PROPPING SAID FRACTURES OPEN;INTRODUCING AND DEPOSITING A LIQUID FUEL SUBSTANTIALLY THRUOUT THEFRACTURES IN THE FRACTURED FORMATION; AND BURNING SAID FUEL THRUOUT SAIDFRACTURES BY CONTACTING SAME WITH AIR AT IGNITION TEMPERATURES INJECTEDTHRU SAID INPUT WELL SO AS TO HEAT THE ADJACENT FORMATION ABOVE ANDBELOW SAID FRACTURES, THEREBY RENDERING THE HYDROCARBONS THEREIN MOREFLUID AND DRIVING SAME INTO SAID OUTPUT WELL; AND RECOVERING PRODUCEDHYDROCARBONS FROM SAID OUTPUT WELL.